Dual wire welding or additive manufacturing system and method

ABSTRACT

A welding or additive manufacturing system includes a contact tip assembly having first and second exit orifices. A wire feeder is configured to deliver a first and second wire electrodes through the exit orifices. An arc generation power supply is configured to output a current waveform to the wire electrodes simultaneously, through the contact tip assembly. The current waveform includes a bridging current portion, and a background current portion having a lower current level than the bridging current portion. The bridging current portion has a current level sufficient to form a bridge droplet between the wire electrodes before the bridge droplet is transferred to a molten puddle during a deposition operation. Solid portions of the wire electrodes do not contact each other during the deposition operation. The bridge droplet is transferred to the molten puddle during a short circuit event between the molten puddle and the wire electrodes.

BACKGROUND OF THE INVENTION Field of the Invention

Devices, systems, and methods consistent with the invention relate tomaterial deposition with a dual wire configuration.

Description of Related Art

When welding, it is often desirable to increase the width of the weldbead or increase the length of the weld puddle. There can be manydifferent reasons for this desire, which are well known in the weldingindustry. For example, it may be desirable to elongate the weld puddleto keep the weld and filler metals molten for a longer period of time soas to reduce porosity. That is, if the weld puddle is molten for alonger period of time there is more time for harmful gases to escape theweld bead before the bead solidifies. Further, it may desirable toincrease the width of a weld bead so as to cover wider weld gap or toincrease a wire deposition rate. In both cases, it is common to use anincreased electrode diameter. The increased diameter will result in bothan elongated and widened weld puddle, even though it may be only desiredto increase the width or the length of the weld puddle, but not both.However, this is not without its disadvantages. Specifically, because alarger electrode is employed more energy is needed in the welding arc tofacilitate proper welding. This increase in energy causes an increase inheat input into the weld and will result in the use of more energy inthe welding operation, because of the larger diameter of the electrodeused. Further, it may create a weld bead profile or cross-section thatis not ideal for certain mechanical applications. Rather than increasingthe diameter of the electrode, it may be desirable to use at least twosmaller electrodes simultaneously.

BRIEF SUMMARY OF THE INVENTION

The following summary presents a simplified summary in order to providea basic understanding of some aspects of the devices, systems and/ormethods discussed herein. This summary is not an extensive overview ofthe devices, systems and/or methods discussed herein. It is not intendedto identify critical elements or to delineate the scope of such devices,systems and/or methods. Its sole purpose is to present some concepts ina simplified form as a prelude to the more detailed description that ispresented later.

In accordance with one aspect of the present invention, provided is awelding or additive manufacturing system. The system includes a contacttip assembly having a first exit orifice and a second exit orifice. Awire feeder is configured to simultaneously deliver a first wireelectrode through the first exit orifice of the contact tip assembly anda second wire electrode through the second exit orifice of the contacttip assembly. An arc generation power supply is configured to output acurrent waveform to the first wire electrode and the second wireelectrode simultaneously through the contact tip assembly. The currentwaveform includes a bridging current portion, and a background currentportion having a lower current level than the bridging current portion.The bridging current portion has a current level sufficient to form abridge droplet between the first wire electrode and the second wireelectrode before the bridge droplet is transferred to a molten puddleduring a deposition operation. Solid portions of the first wireelectrode delivered through the first exit orifice do not contact solidportions of the second wire electrode delivered through the second exitorifice during the deposition operation. The bridge droplet istransferred to the molten puddle during a short circuit event betweenthe molten puddle and the first and second wire electrodes.

In accordance with another aspect of the present invention, provided isa welding or additive manufacturing method. The method includes the stepof providing a current waveform to a contact tip assembly having a firstexit orifice and a second exit orifice. The current waveform includes abridging current portion, and a background current portion having alower current level than the bridging current portion. A first wireelectrode is fed through the first exit orifice of the contact tipassembly, and simultaneously a second wire electrode is fed thorough thesecond exit orifice of the contact tip assembly. During the during thebridging current portion of the current waveform, a bridge droplet isformed coupling the first wire electrode to the second wire electrodebefore the bridge droplet is transferred to a molten puddle during adeposition operation. Solid portions of the first wire electrodedelivered through the first exit orifice do not contact solid portionsof the second wire electrode delivered through the second exit orificeduring the deposition operation. The bridge droplet is transferred tothe molten puddle during a short circuit event between the molten puddleand the first and second wire electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects of the invention will become apparent tothose skilled in the art to which the invention relates upon reading thefollowing description with reference to the accompanying drawings, inwhich:

FIG. 1 illustrates a welding system;

FIG. 2 illustrates a contact tip assembly;

FIGS. 3A to 3C illustrate diagrammatical representations of a weldingoperation;

FIGS. 4A to 4B illustrate diagrammatical representations current andmagnetic field interactions in a deposition operation;

FIG. 5 is an alternative embodiment of a contact tip assembly;

FIG. 6 shows a portion of a welding torch;

FIG. 7 is a perspective view of a contact tip and diffuser;

FIG. 8 is a perspective view of a contact tip;

FIG. 9 is a perspective view of a contact tip;

FIG. 10 is a perspective view of a diffuser;

FIG. 11 is a perspective view of a diffuser;

FIG. 12 is a perspective view of the example drive roll;

FIG. 13 illustrates a cross section of drives rolls feeding dual wires;

FIG. 14 illustrates a diagrammatical representation of an exemplarywelding waveform and wire feeding speeds;

FIG. 15 illustrates a diagrammatical representation of a furtherexemplary welding waveform and wire feeding speeds; and

FIG. 16 is a flow diagram of an example deposition operation.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the invention will now be described below byreference to the attached Figures. The described exemplary embodimentsare intended to assist the understanding of the invention, and are notintended to limit the scope of the invention in any way. Like referencenumerals refer to like elements throughout.

As used herein, “at least one”, “one or more”, and “and/or” areopen-ended expressions that are both conjunctive and disjunctive inoperation. For example, each of the expressions “at least one of A, Band C”, “at least one of A, B, or C”, “one or more of A, B, and C”, “oneor more of A, B, or C” and “A, B, and/or C” means A alone, B alone, Calone, A and B together, A and C together, B and C together, or A, B andC together. Any disjunctive word or phrase presenting two or morealternative terms, whether in the description of embodiments, claims, ordrawings, should be understood to contemplate the possibilities ofincluding one of the terms, either of the terms, or both terms. Forexample, the phrase “A or B” should be understood to include thepossibilities of “A” or “B” or “A and B.”

While embodiments of the present invention discussed herein arediscussed in the context of GMAW type welding, other embodiments of theinvention are not limited thereto. For example, embodiments can beutilized in SAW and FCAW type welding operations, as well as othersimilar types of welding operations. Further, while the electrodesdescribed herein are solid electrodes, again, embodiments of the presentinvention are not limited to the use of solid electrodes as coredelectrodes (either flux or metal cored) can also be used withoutdeparting from the spirit or scope of the present invention. Further,embodiments of the present invention can also be used in manual,semi-automatic and robotic welding operations. Because such systems arewell known, they will not be described in detail herein.

Turning now to the Figures, FIG. 1 depicts an exemplary embodiment of awelding system 100 in accordance with an exemplary embodiment of thepresent invention. The welding system 100 contains an arc generationpower supply, such as welding power source 109, which is coupled to awelding torch 111 via a wire feeder 105. The power source 109 can be anyknown type of welding power source capable of delivering the current andwelding waveform, such as pulse spray, STT and/or short arc type weldingwaveforms. Because the construction, design and operation of such powersupplies are well known, they need not be described in detail herein.The power source 109 outputs welding waveforms to wire electrodes E1 andE2 simultaneously, during a dual wire welding operation, via a contacttip in the welding torch 111. The power source 109 can include acontroller 120 which is coupled to a user interface to allow a user toinput control or welding parameters for the welding operation. Thecontroller 120 can have a processor, CPU, memory etc. to be used tocontrol the operation of the welding process as described herein. Thetorch 111, which can be constructed similar to known manual,semi-automatic or robotic welding torches can be coupled to any known orused welding gun and can be of a straight or gooseneck type. The wirefeeder 105 draws the electrodes E1 and E2 from electrode sources 101 and103, respectively, which can be of any known type, such as reels,spools, containers or the like. The wire feeder 105 is of a knownconstruction and employs drive rolls 107 to draw the electrodes E1 andE2 and push the electrodes to the torch 111. In an exemplary embodimentof the present invention, the drive rolls 107 and wire feeder 105 areconfigured for a single electrode operation. Embodiments of the presentinvention, using a dual wire configuration, can be utilized with a wirefeeder 105 and drive rolls 107 only designed for a single wire feedingoperation. For example, drive rolls 107 can be configured for a single0.045 inch diameter electrode, but will suitable drive two electrodes ofa 0.030 inch diameter without modification to the wire feeder 105 or thedrive rolls 107. Alternatively, the wire feeder 105 can be designed toprovide separate sets of rollers for feeding the electrodes E1/E2respectively, or have drive rolls specifically configured for feedingtwo or more electrodes simultaneously (e.g., via trapezoidal-shaped wirereceiving grooves around the rollers that can accommodate twoelectrodes). In other embodiments, two separate wire feeders can also beused. As shown, the wire feeder(s) 105 is in communication with thepower source 109 consistent with known configurations of weldingoperations.

Once driven by the drive rolls 107, the electrodes E1 and E2 are passedthrough a liner 113 to deliver the electrodes E1 and E2 to the torch111. The liner 113 is appropriately sized to allow for the passage ofthe electrodes E1 and E2 to the torch 111. For example, for two 0.030inch diameter electrodes, a standard 0.0625 inch diameter liner 113(which is typically used for a single 0.0625 inch diameter electrode)can be used with no modification.

Although the examples referenced above discuss the use of two electrodeshaving a same diameter, the present invention is not limited in thisregard as embodiments can use electrodes of a different diameter. Thatis, embodiments of the present invention can use an electrode of afirst, larger, diameter and an electrode of a second, smaller, diameter.In such an embodiment, it is possible to more conveniently weld two workpieces of different thicknesses. For example, the larger electrode canbe oriented to the larger work piece while the smaller electrode can beoriented to the smaller work piece. Further, embodiments of the presentinvention can be used for many different types of welding operationsincluding, but not limited to, metal inert gas, submerged arc, andflux-cored welding. Further, embodiments of the present invention can beused for automatic, robotic and semi-automatic welding operations.Additionally, embodiments of the present invention can be utilized withdifferent electrode types. For example, it is contemplated that a coredelectrode can be coupled with a non-cored electrode. Further, electrodesof differing compositions can be used to achieve the desired weldproperties and composition of the final weld bead. Thus, embodiments ofthe present invention can be utilized in a broad spectrum of weldingoperations.

FIG. 2 depicts an exemplary contact tip assembly 200 of the presentinvention. The contact tip assembly 200 can be made from known contacttip materials and can be used in any known type of welding gun. As shownin this exemplary embodiment, the contact tip assembly has two separatechannels 201 and 203 which run the length of the contact tip assembly200. During welding a first electrode E1 is passed through the firstchannel 201 and the second electrode E2 is passed through the secondchannel 203. The channels 201/203 are typically sized appropriately forthe diameter of wire that is to be passed there through. For example, ifthe electrodes are to have the same diameter the channels will have thesame diameters. However, if different diameters are to be used then thechannels should be sized appropriately so as to properly transfercurrent to the electrodes. Additionally, in the embodiment shown, thechannels 201/203 are configured such that the electrodes E1/E2 exit thedistal end face of the contact tip 200 in a parallel relationship.However, in other exemplary embodiments the channels can be configuredsuch that the electrodes E1/E2 exit the distal end face of the contacttip such that an angle in the range of +/−15° exists between thecenterlines of the respective electrodes. The angling can be determinedbased on the desired performance characteristics of the weldingoperation. It is further noted that in some exemplary embodiments thecontact tip assembly can be a single integrated contact tip with achannels as shown, while in other embodiments the contact tip assemblycan be comprised of two contact tip subassemblies located close to eachother, where the current is directed to each of the contact tipsubassemblies.

As shown in FIG. 2, the respective electrodes E1/E2 are spaced by adistance S which is the distance between the closest edges of theelectrodes. In exemplary embodiments of the present invention, thisdistance is in the range of 0.25 to 4 times the diameter of the largerof the two electrodes E1/E2, while in other exemplary embodiments thedistance S is in the range of 2 to 3 times the largest diameter. Forexample, if each of the electrodes has a diameter of 1 mm, the distanceS can be in the range of 2 to 3 mm. In other exemplary embodiments, thedistance S is in the range of 0.25 to 2.25 times the diameter of one ofthe wire electrodes, such as the larger of the two electrodes. In manualor semi-automatic welding operations the distance S can be in the rangeof 0.25 to 2.25 times the largest electrode diameter, whereas in roboticwelding operations the distance S can be in the same or another range,such as 2.5 to 3.5 times the largest electrode diameter. In exemplaryembodiments, the distance S is in the range of 0.2 to 3.5 mm.

The wire electrodes E1/E2 project from exit orifices on the end face ofthe contact tip 200. The diameter of the exit orifices is slightlylarger than the diameter of the wire electrodes E1/E2. For example, fora 0.035 inch wire, the diameter of the exit orifice could be 0.043inches (1.09 mm); for a 0.040 inch wire, the diameter of the exitorifice could be 0.046 inches (1.17 mm); for a 0.045 inch wire, thediameter of the exit orifice could be 0.052 inches (1.32 mm). Thechannels 201, 203 and exit orifices are spaced appropriately tofacilitate the formation of a single bridge droplet between the wireelectrodes E1/E2 during a deposition operation. For exit orifices sizedfor electrodes having a diameter 0.045 inches and smaller, the distancebetween the exit orifices (inner circumference to inner circumference,similar to distance S) can be less than 3 mm to facilitate the formationof a bridge droplet. However, spacing of 3 mm or greater between theexit orifices may be possible, depending on the wire size, magneticforces, orientation (e.g., angle) of the channels 201, 203, etc. Incertain embodiments, the distance between the exit orifices is withinthe range of 20% to 200% of the diameter of one or both of the exitorifices, which can also correspond to the distance S between the wireelectrodes being in the range of 0.25 to 2.25 times the diameter of theelectrodes.

As explained further below, the distance S should be selected to ensurethat a single bridge droplet is formed between the electrodes E1/E2,before the droplet is transferred to the molten puddle during adeposition operation, while also preventing the solid portions of theelectrodes E1/E2 that are delivered through the exit orifices fromcontacting each other, other than through the bridge droplet.

FIG. 3A depicts an exemplary embodiment of the present invention, whileshowing the interactions of the magnetic forces from the respectiveelectrodes E1 and E2. As shown, due to the flow of current, a magneticfield is generated around the electrodes which tends to create a pinchforce that draws the wires towards each other. This magnetic force tendsto create a droplet bridge between the two electrodes, which will bediscussed in more detail below.

FIG. 3B shows the bridge droplet that is created between the twoelectrodes E1/E2. That is, as the current passing through each of theelectrodes melts the ends of the electrodes the magnetic forces tend todraw the molten droplets at the ends of the electrodes E1/E2 towardseach other until they connect with each other. The distance S is farenough such that the solid portions of the electrodes are not drawn tocontact each other during the deposition operation, but close enoughthat a bridge droplet is created between the electrodes, before themolten droplet is transferred to the weld puddle created by the weldingarc. It can be seen in FIG. 3B that a large bridge droplet can formbetween the electrodes E1/E2; however, there is a relatively small crosssectional area between the droplet and the electrodes. That is, theelectrodes contact the bridge droplet along a small cross sectional areaof the droplet. In certain embodiments, the bridge droplet istransferred to the molten puddle during a short circuit event betweenthe molten puddle and the wire electrodes E1/E2. This process is knownas short arc welding. The short circuit event is depicted in FIG. 3C.The electrodes E1/E2 are driven by the wire feeder toward the workpieceW at a wire feed speed (WFS) that is sufficient to ensure that thebridge droplet does not detach (e.g., pinch off from the electrodes)before the droplet and electrodes short to the molten puddle on theworkpiece W. When the force of feeding the electrodes E1/E2 overcomesthe heating of the arc and the bridge droplet touches the molten puddleas shown, the large bridge droplet is drawn into the weld pool, andsurface tension along with pinch force transfers the droplet from theelectrodes to the weld pool. The size of the bridge droplet issignificantly larger than the droplets formed in conventional, singleelectrode short arc welding. This results in a deposit rate per shortarc cycle that exceeds conventional short arc welding, yet the crosssectional area between the electrodes and the droplet is relativelysmall, requiring less pinch force to transfer that droplet. After thebridge droplet shorts to the molten puddle, the electrodes E1/E2 cancontinue to be driven into the molten puddle for a brief time whilebeing resistive heated by the welding current. Sufficient heat will bepresent in the molten puddle, in combination with the resistive heating,to melt the electrodes E1/E2 and allow them to be consumed into the weldpool as additional filler metal.

FIG. 4A depicts an exemplary representation of current flow in anembodiment of the present invention. As shown, the welding current isdivided so as to flow through each of the respective electrodes andpasses to and through the bridge droplet as it is formed. The currentthen passes from the bridge droplet to the puddle and work piece. Inexemplary embodiments where the electrodes are of the same diameter andtype the current will be essentially divided evenly through theelectrodes. In embodiments where the electrodes have differentresistance values, for example due to different diameters and/orcompositions/construction, the respective currents will be apportioneddue to the relationship of V=I*R, as the welding current is applied tothe contact tip similar to known methodologies and the contact tipprovides the welding current to the respective electrodes via thecontact between the electrodes and the channels of the contact tip. FIG.4B depicts the magnetic forces that aid in creating the bridge droplet.As shown, the magnetic forces tend to pull the respective moltenportions of the electrodes towards each other until they contact witheach other.

FIG. 5 depicts an alternative exemplary embodiment of a contact tip 700that can be used with embodiments of the present invention. As describedpreviously, in some embodiments the electrodes can be directed to thetorch via a single wire guide/liner. Of course, in other embodiments,separate wire guide/liners can be used. However, in embodiments in whicha single wire guide/liner is used, the contact tip can be designed suchthat the electrodes are separated from each other within the contacttip. As shown in FIG. 5, this exemplary contact tip 700 has a singleentrance channel 710 with a single orifice at the upstream end of thecontact tip 700. Each of the electrodes enter the contact tip via thisorifice and pass along the channel 710 until they reach a separationportion 720 of the contact tip, where the separation portion directs oneelectrode into a first exit channel 711 and a second electrode into thesecond exit channel 712, so that the electrodes are directed to theirdiscrete exit orifices 701 and 702, respectively. Of course, thechannels 710, 711 and 712 should be sized appropriately for the size ofelectrodes to be used, and the separation portion 720 should be shapedso as to not scar or scratch the electrodes. As shown in FIG. 5, theexit channels 711 and 712 are angled relative to each other, however, asshown in FIG. 2, these channels can also be oriented parallel to eachother.

As discussed above, the wire electrodes used in a multi-wire depositionoperation (e.g., welding, additive manufacturing, hardfacing, etc.) canbe spaced by a distance S that facilitates formation of a bridge dropletbetween the wire electrodes. The size of the bridge droplet isdetermined by the spacing between the wire electrodes and the spacingbetween the exit orifices in the contact tip. The size of the bridgedroplet determines the width of the electric arc that exists during thedeposition operation, and reducing the spacing between the exit orificesand wire electrodes narrows the arc width. Larger bridge droplets may bepreferred for larger welds, and smaller bridge droplets preferred forsmaller welds. Deposition rate is impacted by the arc width, and thedeposition rate for small gauge wires can be increased by reducing thespacing between the exit orifices and wire electrodes (e.g., fromapproximately 2 mm to 1 mm).

The maximum spacing between the exit orifices and between the wireelectrodes is reached when the magnetic forces developed by the currentwaveform (e.g., at the peak current level) still allow formation of thebridge droplet, and is exceeded when bridging is no longer possible. Theminimum spacing is that which keeps the solid portions of the wiresseparated at the point of bridging. The magnetic forces tend to pull thewire electrodes together, and the wires are somewhat flexible. Thus, theminimum spacing between the exit orifices and between the wireelectrodes will depend on the stiffness of the electrodes, which isimpacted by parameters such as wire diameter, material of construction,etc.

FIG. 6 depicts an end portion of an exemplary welding torch inaccordance with the present invention. Because the construction andoperation of welding torches is generally known, the details of suchconstruction and operation will not be discussed in detail herein. Asshown, the torch includes a number of components and is used to deliverat least two wire electrodes and a shielding gas to a workpiece for awelding or additive manufacturing operation. The torch includes adiffuser 205 which aids in properly directing and distributing theshielding gas for a welding operation. Coupled to the downstream end ofthe diffuser 205 is a contact tip 200, which is used to pass the weldingcurrent into the at least two wire electrodes which are passing thoughthe contact tip simultaneously during welding. The contact tip 200 isconfigured to facilitate the formation of a bridge droplet between thewire electrodes that are delivered through bores or channels in thecontact tip. The bridge droplet couples the first wire electrode to thesecond wire electrode prior to contacting a molten puddle created by thedeposition operation, as discussed above.

Threaded onto the outside of the diffuser 205 is an insulator 206. Theinsulator 206 electrically isolates a nozzle 204 from the electricallylive components within the torch. The nozzle 204 directs the shieldinggas from the diffuser 205 to the distal end of the torch and theworkpiece during welding.

Conventional contact tips have threads on an upstream or proximal end ofthe contact tip that thread into the diffuser. The contact tip anddiffuser are connected by screwing the contact tip into the diffuser.Such a fastening system works well for welding with single wires. Thewelding wire can be threaded through the contact tip and the contact tipcan be rotated around the wire multiple times and screwed into thediffuser. However, when welding with multiple welding wiressimultaneously passing through the contact tip, such a fastening systemwould result in an undesirable twisting of the welding wires. Forexample, if two welding wires are passed through the contact tip,subsequently threading the contact tip onto the diffuser by multipleturns requiring greater than 360° of rotation will result in the weldingwires becoming twisted and unable to be fed through the contact tip.

The contact tip 200 in FIG. 6 is attached to the diffuser 205 byrotation of the contact tip through less than 360°, such as 270°(three-quarter turn), 180° (one-half turn), 90° (quarter turn), lessthan 90°, etc. The rotation of the contact tip 200 necessary to attachthe contact tip to the diffuser 205 can be any angle as desired that ispreferably less than 360° and results in the multiple wire electrodespassing through the contact tip not becoming unduly twisted duringinstallation of the contact tip. If the welding wires are unduly twistedduring installation of the contact tip, wire feeding problems willresult and “bird nesting” of the welding wires can occur.

With reference to FIGS. 6-11, the contact tip 200 is attached to thediffuser 205 by a quarter turn, clockwise rotation of the contact tipwithin the diffuser. The contact tip 200 has a forward or downstreamdistal portion that has a tapered shape and includes flats 215 toaccommodate gripping by a tool, such as pliers. The contact tip 200 hasa rearward or upstream proximal portion 208 that is generallycylindrical, but includes a radially-projecting tab 210 that engages aslot 212 in an interior wall of the diffuser 205, to securely connectthe contact tip to the diffuser. The rearward portion 208 of the contacttip 200 is located within the diffuser 205 when the contact tip isinstalled onto the diffuser, and acts as a mounting shank for thecontact tip. It can be seen that the diameter of the rearward portion208 of the contact tip is smaller than the adjacent downstream portion,which results in a shoulder 211 projecting radially from the cylindricalrearward portion 208 of the contact tip. The shoulder 211 seats againstthe terminal end face of the diffuser 205 when the contact tip 200 isinstalled onto the diffuser.

The contact tip 200 can be made from known contact tip materials and canbe used in any known type of welding gun. The contact tip can comprisean electrically-conductive body, such as copper, extending from itsrearward, proximal end to its forward, distal end. As shown in thisexemplary embodiment, the contact tip 200 has two separate wire channelsor bores 214 and 216 which run the length of the contact tip. Thechannels 214/216 can extend between wire entrance orifices on theproximal end face of the mounting shank 208, and wire exit orifices onthe distal end face of the contact tip. During welding, a first wireelectrode is delivered through the first channel 214 and a second wireelectrode is delivered through the second channel 216. The channels214/216 are typically sized appropriately for the diameter of wire thatis to be fed through the channel. For example, if the electrodes are tohave the same diameter, then the channels will have the same diameters.However, if different wire sizes are to be used together, then thechannels should be sized appropriately so as to properly transfercurrent to the differently-sized electrodes. Additionally, in theembodiment shown, the channels 214/216 are configured such that theelectrodes exit the distal end face of the contact tip 200 in a parallelrelationship. However, in other exemplary embodiments the channels canbe configured so that the electrodes exit the distal end face of thecontact tip such that an angle in the range of +/−15° exists between thecenterlines of the respective electrodes. The angling can be determinedbased on the desired performance characteristics of the weldingoperation. The example contact tips discussed herein are shown havingtwo electrode bores. However, it is to be appreciated that the contacttips could have bores for more than two electrodes, such as three ormore bores.

The slot 212 in the interior wall of the diffuser 205 includes an axialportion 218 and a helical portion 220. The axial portion 218 of the slot212 extends to the downstream terminal end face of the diffuser 205,against which the shoulder 211 of the contact tip 200 seats. After thewelding electrodes are fed through the contact tip 200, theradially-projecting tab 210 on the mounting shank 208 is inserted intothe axial portion 218 of the 212 slot and the contact tip is pushed intothe diffuser 205. When the tab 210 reaches the helical portion 220 ofthe slot, the contact tip 200 is rotated to move the tab to the end ofthe helical portion. The helical portion 220 has a slight upstream pitchthat draws the contact tip 200 inward as the contact tip is rotated, sothat the shoulder 211 of the contact tip seats against the downstreamterminal end face of the diffuser 205. The tab 210 on the mounting shank208 can have a tapered edge 217 that matches the pitch of the slot 212in the diffuser 205, to help ensure a tight connection between the twocomponents. In the example embodiment shown, the helical portion 220 ofthe slot 212 allows for a quarter turn of the contact tip 200 to securethe contact tip to the diffuser 205. However, it is to be appreciatedthat other rotational angles are possible (e.g., more or less than aquarter turn or 90°). For example, the helical portion 220 of the slotcan extend less than 360° around the inner circumference of the interiorchamber of the diffuser 205.

FIG. 12 illustrates an example drive roll 107 for use in a wire feederin a dual wire welding system. The drive roll 107 has a central bore.The inner surface of the bore can include contoured recesses 131 forreceiving projections on a driving mechanism, such as a drive gear, totransfer drive torque to the drive roll 107. The drive roll 107 includesone or more annular or circumferential wire receiving grooves 133, 135.The wire receiving grooves 133, 135 are spaced axially along thecircumference of the drive roll 107. The wire receiving grooves 133, 135are designed to receive two welding wires. Example standard welding wirediameters for use with the drive rolls 107 include 0.030 inches, 0.035inches, 0.040 inches, 0.045 inches, etc. The wire receiving grooves 133,135 can have the same width and depth as each other, or have differentwidths and depths to accommodate different sizes or combinations of dualwelding wires. If the wire receiving grooves 133, 135 each have the samewidth and depth, then the drive roll 107 can be reused when one grooveis worn out by simply flipping the drive roll over and reinstalling iton the wire feeder. The wire receiving grooves 133, 135 can beconfigured to simultaneously drive two wires having the same diameter,or two wires having different diameters. The wire receiving grooves 133,135 can have a generally trapezoidal shape with straight, angled orinwardly-tapered sidewalls and a flat base extending between thesidewalls. However, the wire receiving grooves 133, 135 could have othershapes besides a trapezoidal shape, such as having a curved, concavegroove base for example. In certain embodiments, the grooves 133, 135can include knurling or other frictional surface treatments to help gripthe welding wires.

FIG. 13 shows a partial cross section of two drive rolls 107 as theywould be mounted on a wire feeder for supplying dual welding wiresduring a deposition operation. The drive rolls 107 are biased togetherto provide a clamping force on the first E1 and the second E2 weldingwires. The welding wires E1, E2 are both located in the annular groovesof the upper and lower drive rolls 107. Due to the bias force applied tothe drive rolls 107, the welding wires E1, E2 are clamped in the annulargrooves between upper and lower sidewalls 150 forming the grooves andthe neighboring welding wire. The welding wires E1, E2 are stably heldvia three points of contact within the annular grooves. This clampingsystem can allow both wires to be fed through the wire feeder in aconsistent manner. The two welding wires E1, E2 support each otherduring feeding and pull each other along via friction. Because thesidewalls 150 of the annular grooves are angled, they apply bothvertical and horizontal clamping forces on the welding wires E1, E2. Thehorizontal clamping force pushes the welding wires E1, E2 together,causing them to contact each other. The welding wires E1, E2 are clampedwithin the annular grooves so as to be radially offset from both concavegroove bases 152. That is, the welding wires E1, E2 are pinned betweeneach other and the angled sidewalls 150 of the grooves such that gapsexist between each of the welding wires and the groove bases 152. In anexample embodiment, the angle between the sidewalls 150 and the outercircumference of the drive roll 107 is about 150°, although other anglesare possible and can be determined with sound engineering judgment.

The wire clamping provided by the drive rolls 107 allows for somevariability (e.g., due to manufacturing tolerances) in the diameters ofthe welding wires E1, E2. If each welding wire E1, E2 had its owndedicated annular groove in the drive rolls 107, and one of the weldingwires was slightly larger than the other, then the smaller welding wiremight not be adequately clamped between the drive rolls. In such asituation, the larger welding wire would limit the radial displacementof the drive rolls 107 toward each other, thereby preventing properclamping of the smaller wire. This could lead to feeding problems andso-called birdnesting of the smaller welding wire during feeding. Theclamping system discussed above can accommodate wires of different sizesbecause the clamping system is self-adjusting. When one welding wire E1is larger than the other E2, the contact point between the wires isshifted axially from a central position within the annular groovestoward the smaller wire. Three points of contact are maintained on eachwelding wire E1, E2 by the sidewalls 150 of the groove and theneighboring welding wire.

FIGS. 14 and 15 provide example current 300 and voltage 320 waveformsfor a short arc welding operation using a bridge droplet as describedabove. FIGS. 14 and 15 also show example changes to wire feed speed(WFS) 340, 360 during the short arc welding operation, and movement ofthe wire electrodes and the bridge droplet during the welding operation.The periodic short circuit event, during which the bridge droplet istransferred to the molten weld puddle and the wire electrodes areshorted to the workpiece, is indicated in FIGS. 14 and 15 as theinterval between “short” and “clear”. After the short circuit event iscleared, a new bridge droplet is formed between the wire electrodeswhile the electrodes are separated from the weld puddle and workpiece.The electrodes and newly-developed bridge droplet are driven toward theweld puddle by the wire feeder, to transfer another bridge droplet tothe weld puddle via another short circuit event. This cycle repeats forthe duration of the short arc deposition operation.

The current waveform 300 includes a bridging current portion 302. Thebridging current portion 302 is a high current portion of the waveformand is sufficient to form the bridge droplet between the wireelectrodes. An example current range for the bridging current portion302 is 200 A-1000 A. After the bridging current portion 302, the currentcan be lowered to a background current portion 304. Example currentlevels for the background current portion are less than 100 A, less than50 A, and about 20 A. The lower background current portion 304 reducesthe magnetic forces around the bridge droplet and helps to keep thebridge droplet from detaching from the electrodes before a short circuitevent occurs (i.e, before the bridge droplet shorts to the moltenpuddle). During the bridging current portion 302 and the backgroundcurrent portion 304, the WFS is positive (i.e., the wire electrodes aredriven toward the molten puddle), as shown. The wire feed speed can bekept constant during the bridging current portion 302 and the backgroundcurrent portion 304, or the WFS be adjusted during these portions of thewelding waveform. For example, the WFS can be higher during the bridgingcurrent portion 302 and then reduced as the current waveform 300transitions to the background current 304. The WFS can also bedecreasing (e.g., have a negative slope) as the bridge droplet shorts tothe molten puddle for a soft or gentle contact when the short occurs.Regardless of whether the WFS is kept constant or altered while theelectrodes and bridge droplet are driven toward the molten puddle, theWFS should be high enough to ensure that the bridge droplet is quicklypushed into the molten puddle before it separates from the electrodes.

The current waveform 300 can include a short circuit wire heatingportion 306. The short circuit wire heating portion 306 can have ahigher current level than the background current portion 304 and a lowercurrent level than the bridging current portion 302. An example currentlevel for the short circuit wire heating portion 304 is about 100 A,although higher or lower current levels are possible. For example, theshort circuit wire heating portion 304 could be about the same level asthe background current portion 304. The short circuit wire heatingportion 306 occurs during the short circuit event, after the bridgedroplet shorts into the molten puddle. The short circuit wire heatingportion 306 serves to resistive heat the electrodes, which are alsoshorted to the molten puddle, while the arc is extinguished. Theresistive heating combined with the heat present in the molten puddleallows the electrodes to be consumed as filler wire in the molten puddleas they are driven into the puddle by the wire feeder. The total heatinput of the short circuit wire heating portion 306 is low due to thelow voltage of the short circuit. The WFS can remain positive or forwardduring the majority of the short circuit wire heating portion 306, butcould be slowed, stopped or even reversed during the short circuit wireheating portion if desired. The short circuit wire heating portion 306of the current waveform 300 can be applied for a predetermined durationduring the short circuit event, and then the current level can bereduced to a low level 308 before the short clears, to minimize spatter.Rather than applying the short circuit wire heating portion 306 for apredetermined duration, this intermediate current level could bemaintained until it is apparent that the short is about to clear, asdetermined by the welding power source or wire feeder, at which time thecurrent level can be reduced to the low level 308. The short circuitwire heating portion 306 can also be regulated as a heating power levelthat is maintained for a duration that is sufficient to supply a desiredamount of energy (e.g., joules) through the electrodes. The power levelcan be integrated to determine the total amount of energy suppliedduring the short circuit wire heating portion 308. When the desiredamount of heating energy has been supplied, the current level can bereduced to the low level 308. After the welding power source or wirefeeder determines that the short has cleared, or after the low currentlevel 308 has been applied for a duration during which the short isexpected to clear, the power source switches from the spatter minimizinglow current level 308 to the bridge droplet forming bridging currentlevel 302.

The welding power source and/or wire feeder can detect the occurrence ofthe short circuit event by monitoring the voltage of the depositionoperation (e.g., the welding voltage). In certain embodiments, thewelding power source can reduce the welding current upon the occurrenceof the short circuit event, to reduce spatter. Such a current reductionis shown by dashed lines on the current trace of FIG. 14. The weldingpower source and/or wire feeder can also determine when the short isabout to clear, such as by monitoring the rate of change of the weldingvoltage (dV/dt) for example. Based on the rate of change of the weldingvoltage (dV/dt), WFS can be adjusted and the bridging current level 302output to the electrodes, in response to the short being clearing.

During arcing portions of the deposition operation, the WFS is positiveand relative high to push the bridge droplet into the molten puddlebefore it detaches from the electrodes. However, during the shortcircuit event, the WFS can be reduced to allow a pinch force from thecurrent flow to separate the electrodes from the molten puddle andreestablish arcs from the electrodes. In certain embodiments, wirefeeding can be stopped during the short circuit event, as shownschematically in FIG. 15 with the WFS reaching zero during the shortcircuit event. The feeding direction of the wire feeder can also bereversed during the short circuit event (FIG. 14), to pull theelectrodes out of the molten puddle. In that case, the wire feederinitially drives the electrodes into the molten puddle during the shortcircuit event to add additional filler metal to the puddle, and thensubsequently pulls the electrodes away from the molten puddle to helpreestablish the arcs. The timing for slowing, stopping, and/or reversingthe electrodes can be based on the detected occurrence of the shortcircuit event. For example, the power source can monitor the weldingvoltage and control the wire feeder to slow/stop/reverse the electrodeswhen the welding voltage drops to a short circuit level, orslow/stop/reverse the electrodes at a predetermined duration of timeafter the short circuit occurs. The welding arcs between the electrodesand the molten puddle can be reestablished while the wire feeding isslowed or stopped or the feeding direction is reversed (away from themolten puddle), as shown at the bottom of FIGS. 14 and 15. The wirefeeder can then resume delivering the electrodes toward the moltenpuddle after the short clears and the arcs are reestablished as shown.Changes to the WFS and direction can be coordinated with changes to thecurrent level during the short circuit and arcing portions of thedeposition operation.

The frequency of the welding waveforms discussed above can be lower thanconventional welding waveforms while maintaining high deposition rates.A conventional welding waveform may have a frequency of about 100 Hz,for example, whereas the waveforms of the present invention may be lessthan 100 Hz, such between 20 Hz and 60 Hz.

FIG. 16 depicts a flow diagram of an example deposition operation, whichmay be welding, additive manufacturing, etc. A current waveform isprovided to a contact tip assembly having a first exit orifice and asecond exit orifice 400. The current waveform includes at least abridging current portion and a background current portion having a lowercurrent level than the bridging current portion. A first wire electrodeis fed through the first exit orifice of the contact tip assembly andsimultaneously a second wire electrode is fed thorough the second exitorifice of the contact tip assembly 402. The electrodes are fed by oneor more wire feeders, and the feeding speeds can be varied. During thebridging current portion of the current waveform, a bridge droplet isformed before the bridge droplet is transferred to a molten puddleduring a deposition operation 404. The bridge droplet couples the firstwire electrode to the second wire electrode, and solid portions of thefirst wire electrode delivered through the first exit orifice do notcontact solid portions of the second wire electrode delivered throughthe second exit orifice. The bridge droplet is transferred to the moltenpuddle during a short circuit event between the molten puddle and thefirst and second wire electrodes 406. A feed speed of the first andsecond wire electrodes is reduced and/or the feeding direction reversedduring the short circuit event 408. In certain embodiments, the firstand second wire electrodes are initially driven into the molten puddleand subsequently pulled away from the molten puddle during the shortcircuit event. Feeding of the first and second wire electrodes can alsobe stopped during the short circuit event, and resumed when the short iscleared. A rate of change of voltage (dV/dt) of the deposition operationcan be monitored to determine clearance of the short circuit event fromthe rate of change of voltage (dV/dt) 410. Arcs are reestablishedbetween the molten puddle and the first and second wire electrodes 412.The arcs can be reestablished while feeding of the electrodes is slowedor stopped, or while the feeding direction of the electrodes is reversed(i.e., away from the molten puddle).

The use of embodiments described herein can provide significantimprovements in stability, weld structure and performance over knownwelding operations. However, in addition to welding operations,embodiments can be used in additive manufacturing operations. In factthe system 100 described above can be used in additive manufacturingoperations as well as welding operations. In exemplary embodiments,improved deposition rates can be achieved in additive manufacturingoperations using the dual wire short arc techniques described above.Because additive manufacturing processes and systems are known, thedetails of such processes and systems need not be described herein.

It is noted that exemplary embodiments are not limited to the usage ofthe waveforms discussed above and described herein, as other weldingtype waveforms can be used with embodiments of the present invention.For example, other embodiments can use variable polarity or ACwaveforms, etc. without departing from the spirit and scope of thepresent invention. For example, in variable polarity embodiments thebridging current portion of the welding waveform can be done in anegative polarity such that the bridge droplet is created while reducingthe overall heat input into the weld puddle.

Embodiments of the present invention can be used with different typesand combinations of consumables including flux cored consumables. Infact, embodiments of the present invention can provide a more stablewelding operation when using flux cored electrodes. Specifically, theuse of a bridging droplet can aid in stabilizing flux core droplets thatcan tend to be unstable in a single wire welding operation. Further,embodiments of the present invention allow for increased weld and arcstability at higher deposition rates, such as those exceeding 10 lb/hr.

Additionally, as indicated above the consumables can be of differenttypes and/or compositions, which can optimize a given welding operation.That is, the use of two different, but compatible, consumables can becombined to create a desired weld joint. For example, compatibleconsumables include hardfacing wires, stainless wires, nickel alloys andsteel wires of different composition can be combined. As one specificexample a mild steel wire can be combined with an overalloyed wire tomake a 309 stainless steel composition. This can be advantageous when asingle consumable of the type desired does not have desirable weldproperties. For example, some consumables for specialized weldingprovide the desired weld chemistry but are extremely difficult to useand have difficulty providing a satisfactory weld. However, embodimentsof the present invention allow for the use of two consumables that areeasier to weld with to be combined to create the desired weld chemistry.Embodiments of the present invention can be used to create analloy/deposit chemistry that is not otherwise commercially available, orotherwise very expensive to manufacture. Thus, two different consumablescan be used to obviate the need for an expensive or unavailableconsumable. Further, embodiments can be used to create a diluted alloy.For example, a first welding wire could be a common inexpensive alloyand a second welding wire could be a specialty wire. The desired depositwould be the average of the two wires, mixed well in the formation ofthe bridged droplet, at the lower average cost of the two wires, over anexpensive specialty wire. Further, in some applications, the desireddeposit could be unavailable due to the lack of appropriate consumablechemistry, but could be reached by mixing two standard alloy wires,mixed within the bridged droplet and deposited as a single droplet.Further, in some applications, such as the application of wearresistance metals, the desired deposit may be combination of tungstencarbide particles from one wire and chrome carbide particles fromanother. Still in another application, a larger wire housing largerparticles within is mixed with a smaller wire containing fewer particlesor smaller particles, to deposit a mixture of the two wires. Here theexpected contribution from each of the wires is proportional to the sizeof wire given the wire feed speeds are same. In yet another example, thewire feed speeds of the wires are different to allow the alloy producedto change based on the desired deposit but the mixing of the wires isstill produced by the bridge droplet created between the wires.

It should be evident that this disclosure is by way of example and thatvarious changes may be made by adding, modifying or eliminating detailswithout departing from the fair scope of the teaching contained in thisdisclosure. The invention is therefore not limited to particular detailsof this disclosure except to the extent that the following claims arenecessarily so limited.

What is claimed is:
 1. A welding or additive manufacturing system,comprising: a contact tip assembly having a first exit orifice and asecond exit orifice; a wire feeder configured to simultaneously delivera first wire electrode through the first exit orifice of the contact tipassembly and a second wire electrode through the second exit orifice ofthe contact tip assembly; and an arc generation power supply configuredto output a current waveform to the first wire electrode and the secondwire electrode simultaneously through the contact tip assembly, wherein:the current waveform includes a bridging current portion, and abackground current portion having a lower current level than thebridging current portion, the bridging current portion has a currentlevel sufficient to form a bridge droplet between the first wireelectrode and the second wire electrode before the bridge droplet istransferred to a molten puddle during a deposition operation, whereinsolid portions of the first wire electrode delivered through the firstexit orifice are spaced apart from solid portions of the second wireelectrode delivered through the second exit orifice during thedeposition operation, and the bridge droplet is transferred to themolten puddle during a short circuit event between the molten puddle andthe first and second wire electrodes.
 2. The welding or additivemanufacturing system of claim 1, wherein the current waveform includes ashort circuit wire heating portion that occurs during the short circuitevent, wherein the short circuit wire heating portion has a highercurrent level than the background current portion and a lower currentlevel than the bridging current portion.
 3. The welding or additivemanufacturing system of claim 1, wherein, during the short circuitevent, a feeding direction of the wire feeder is reversed such that thefirst wire electrode and the second wire electrode are initially driveninto the molten puddle and subsequently pulled away from the moltenpuddle by the wire feeder during the short circuit event.
 4. The weldingor additive manufacturing system of claim 3, wherein arcs arereestablished between the molten puddle and the first and second wireelectrodes while the feeding direction is away from the molten puddle.5. The welding or additive manufacturing system of claim 1, wherein,during the short circuit event, a wire feed speed of the wire feeder isreduced.
 6. The welding or additive manufacturing system of claim 5,wherein the wire feeder stops feeding the first wire electrode and thesecond wire electrode toward the molten puddle during the short circuitevent, and resumes feeding the first wire electrode and the second wireelectrode toward the molten puddle when the short circuit event iscleared.
 7. The welding or additive manufacturing system of claim 5,wherein one or both of the wire feeder and the arc generation powersupply is configured to monitor a rate of change of voltage (dV/dt) ofthe deposition operation and determine clearance of the short circuitevent from said rate of change of voltage (dV/dt).
 8. The welding oradditive manufacturing system of claim 7, wherein the arc generationpower supply is configured to output the bridging current portion of thewelding waveform based on the clearance of the short circuit event. 9.The welding or additive manufacturing system of claim 1, wherein thewire feeder delivers the first wire electrode and the second wireelectrode toward the molten puddle at a first wire feed speed during thebridging current portion of the current waveform, and delivers the firstwire electrode and the second wire electrode toward the molten puddle ata second wire feed speed during the background current portion of thecurrent waveform, wherein the second wire feed speed is less than thefirst wire feed speed.
 10. The welding or additive manufacturing systemof claim 1, wherein the first and second exit orifices are separatedfrom each other by a distance configured to facilitate formation of thebridge droplet between the first wire electrode and the second wireelectrode.
 11. A welding or additive manufacturing method, comprisingthe steps of: providing a current waveform to a contact tip assemblyhaving a first exit orifice and a second exit orifice, wherein thecurrent waveform includes a bridging current portion, and a backgroundcurrent portion having a lower current level than the bridging currentportion; feeding a first wire electrode through the first exit orificeof the contact tip assembly and simultaneously feeding a second wireelectrode thorough the second exit orifice of the contact tip assembly;forming, during the bridging current portion of the current waveform, abridge droplet coupling the first wire electrode to the second wireelectrode before the bridge droplet is transferred to a molten puddleduring a deposition operation, wherein solid portions of the first wireelectrode delivered through the first exit orifice are spaced apart fromsolid portions of the second wire electrode delivered through the secondexit orifice during the deposition operation; and transferring thebridge droplet to the molten puddle during a short circuit event betweenthe molten puddle and the first and second wire electrodes.
 12. Thewelding or additive manufacturing method of claim 11, wherein thecurrent waveform includes a short circuit wire heating portion thatoccurs during the short circuit event, wherein the short circuit wireheating portion has a higher current level than the background currentportion and a lower current level than the bridging current portion. 13.The welding or additive manufacturing method of claim 11, furthercomprising the step of reversing a feeding direction of the first andsecond wire electrodes during the short circuit event such that thefirst and second wire electrodes are initially driven into the moltenpuddle and subsequently pulled away from the molten puddle during theshort circuit event.
 14. The welding or additive manufacturing method ofclaim 13, further comprising the step of reestablishing arcs between themolten puddle and the first and second wire electrodes while the feedingdirection of the first and second wire electrodes is away from themolten puddle.
 15. The welding or additive manufacturing method of claim11, further comprising the step of reducing a wire feed speed of thefirst and second wire electrodes during the short circuit event.
 16. Thewelding or additive manufacturing method of claim 15, further comprisingthe steps of stopping the feeding of the first and second wireelectrodes during the short circuit event, and resuming the feeding ofthe first and second wire electrodes when the short circuit event iscleared.
 17. The welding or additive manufacturing method of claim 15,further comprising the step of monitoring a rate of change of voltage(dV/dt) of the deposition operation and determining clearance of theshort circuit event from said rate of change of voltage (dV/dt).
 18. Thewelding or additive manufacturing method of claim 17, wherein a timingof the bridging current portion of the welding waveform is based ondetermining the clearance of the short circuit event.
 19. The welding oradditive manufacturing method of claim 11, wherein the step of feedingincludes feeding the first wire electrode and the second wire electrodetoward the molten puddle at a first wire feed speed during the bridgingcurrent portion of the current waveform, and feeding the first wireelectrode and the second wire electrode toward the molten puddle at asecond wire feed speed during the background current portion of thecurrent waveform, wherein the second wire feed speed is less than thefirst wire feed speed.
 20. The welding or additive manufacturing methodof claim 11, wherein the first and second exit orifices are separatedfrom each other by a distance configured to facilitate formation of thebridge droplet between the first wire electrode and the second wireelectrode.